III. To analyze and model erlotinib's pharmacokinetic/pharmacodynamic (PK/PD) profile. Serial blood samples will be drawn at the beginning and at the end of erlotinib treatment, and pharmacokinetic parameters will be determined. The status of EGFR genotype (and that of others genes linked to erlotinib PK/PD) clinical toxicity, and dose will be examined as possible other influential covariates by comparing them to experimentally measured PK profiles, and PD profiles (in particular, the pEGFR/EGFR ratio). The goal of these studies will be to determine the optimal biologic concentration (OBC) of Erlotinib that is associated with the lowest toxicity and highest effect, for a given subject's pharmacogenomic profile.